BONE-MARROW DOSIMETRY - REGIONAL VARIABILITY OF MARROW-LOCALIZING ANTIBODY

In radiolabeled antibody therapy, imaging and biopsy-based methods are used to estimate marrow activity concentration when the administered antibody localizes to the marrow. Absorbed dose estimates obtained usi ng such measurements may be subject to large variability due to the po tential for regional differences in marrow activity concentration, Thi s variability was examined in ten patients with leukemia after adminis tration of I-131-labeled HuM195 antibody, Methods: Regions of interest were drawn around the head and neck of the humerus and femur (both si des) and around lumbar vertebra 3 (L3) and 4 (L4) on a series of plana r images collected at multiple times postadministration of the antibod y, A single exponential fit to each attenuation-corrected, time-activi ty curve was obtained to estimate clearance half-life and the back-ext rapolated percent injected activity. Results: The activity concentrati on in the femoral head and neck (mean and s.d. = 0.04 +/- 0.02% ID/g) was not significantly different than that measured in L3 and L4 (0.06 +/- 0.02% ID/g) but was significantly lower than the concentrations me asured in the humeral head and neck regions (0.07 +/- 0.03% ID/g, p < 0.05), Although half-life estimates differing by more than a factor of 2 were observed among different regions for individual patients, no s ystematic difference was observed in half-life between regions overall , S-factors were used for individual marrow regions to determine the m ean absorbed dose to marrow in the femoral and humeral heads and the l umbar vertebrae (L3 and L4) which were 0.66 +/- 0.3, 1.0 +/- 0.3 and 2 .2 +/- 0.5 mGy/MBq (2.4, 3.8 and 8.3 rad/mCi), respectively, Conclusio n: A single value is generally quoted for the absorbed dose delivered to the red marrow following marrow-localizing radiolabeled antibody ad ministration, These results suggest that the regional marrow dose may differ significantly from the mean.